I'm kind of with Annie on the thank you cards thing - while it sounds like a lovely idea, and I can think of a number of recent times when drivers have done nice things I'd have liked to be able thank them for, I just don't see how trying to hand them little cards could work well in practice. In most of the circumstances when you'd want to give one to someone, you're both out in traffic, in motion, and there's really no good way to do it. I think usually a smile and wave is a little more practical.
BTW, my favourite nice-driver story lately: I was on Davenport trying to turn left onto Symington, and I'd gone into the left turn lane because traffic didn't seem too heavy and often at that intersection I can actually manage to turn left "properly". But as soon as I was there, it seemed like traffic coming the other way got heavier, so I couldn't actually turn, and was stuck there through a couple of changes of the light - and a car came up behind me, also wanting to turn left. After a minute the driver behind me rolled down his window and popped his head out, and I sort of braced myself to get yelled at, since I'm sure he wasn't very happy to be stuck behind me. But instead, he called out in a friendly tone "Try moving a little further out into the intersection - they'll be able to see you better and someone might let you through. And if you do get a break, you'll be able to take advantage of it easier." So I did, and it worked - we both got through just before the light turned yellow again. It was just such a nice surprise to not only have a driver be actively helpful, but to have him do that when I was expecting him to be angry. (And yes, I did call out "Thank you!" as he passed.)